Processes for the separation of undesired accompanying substances from a raw synthesis gas stream are known, such as e.g. the Rectisol process. The Rectisol process is described in principle in Ullmann's Encyclopedia of Industrial Chemistry, 6th Edition, Vol. 15, p. 399 ff. This process serves to purify raw synthesis gas chiefly consisting of CO and H2, which was produced from coal or coke by the fixed-bed pressure gasification process, by absorption of the accompanying gases. The fixed-bed pressure gasification process is described in principle in Ullmann's Encyclopedia of Industrial Chemistry, 6th Edition, Vol. 15, p. 367 ff The Rectisol process uses methanol as absorbent, utilizing the property of methanol that its absorption capacity for the accompanying substances greatly increases with decreasing temperature, while it remains virtually constant for CO and H2. The undesired accompanying substances chiefly are the accompanying gases carbonyl sulfide (COS), hydrogen sulfide (H2S) and carbon dioxide (CO2).
In this process, the methanol used as absorbent is circulated via regeneration plants. In the regeneration plants, the loaded methanol is liberated from the absorbed gases in a physical way. In a first regeneration step CO2 is removed from the loaded methanol absorbent by depressurization and/or stripping with a gas, for example nitrogen. In a second regeneration step the sulfur-containing gases, COS and H2S, are expelled by heating. It is aimed to produce a COS/H2S gas largely free from CO2, as its economically interesting further processing is impaired by mixing with CO2.
In the Rectisol process a distinction is made between the standard and the selective Rectisol process.
In the so-called standard Rectisol process the accompanying gases COS/H2S and the CO2 jointly are separated from the raw synthesis gas in one absorption step. In the so-called selective Rectisol process the sulfur-containing accompanying gases COS/H2S and the CO2 each are separated from the raw synthesis gas in separate absorption steps proceeding one after the other. This selective absorption becomes possible by suitable adjustment of the process parameters, in particular the quantity ratio of absorbent and gas to be absorbed. The advantage of the selective absorption consists in that the COS/H2S gas and the CO2 gas for the most part already are kept separate during the absorption and only the smaller part must be separated during the regeneration of the methanol.
Beside the above-mentioned components COS, H2S and CO2 the raw synthesis gas however contains further accompanying gases. The document DE 10 2006 056 117 A1 describes a corresponding process, in which before the absorption steps for the separation of the sulfur components and the CO2 a further absorption step is carried out, in order to selectively separate metal carbonyls from the synthesis gas.
The raw synthesis gas often also contains higher hydrocarbons and mercaptans, in particular methyl mercaptan, and other substances, such as aromatics, in particular benzene, NH3, HCN, thiophene, other organic sulfur compounds and lighter hydrocarbons in such quantities that they lead to problems in the conventional selective Rectisol process. This is the case in particular when the raw synthesis gas has been obtained by gasification of coal, for example by the fixed-bed pressure gasification process. The problems consist in that these substances either are enriched in the absorbent and hence render it useless after a short time of use, or, especially in the case of mercaptans, that during the regeneration of the absorbent by depressurization or stripping they are expelled together with the CO2 and possibly represent an odor nuisance to the environment.